Main shafts of a gas turbine and a turbocharger are driven to rotate at high speed. Further, turbine blades mounted to the main shafts are exposed to high temperature. Thus, bearings for supporting those main shafts are required to endure severe environments involving high temperature and high speed rotation. As bearings for such use, an oil-lubricated rolling bearing or a hydraulic dynamic pressure bearing may be used. However, use of those bearings is restricted under such conditions that lubrication with a liquid such as a lubricating oil is difficult, that an auxiliary device of a lubricating oil circulatory system is difficult to arrange separately in view of energy efficiency, and that shearing resistance of the liquid causes problems. Under the circumstance, attention has been focused on an air dynamic pressure bearing as a bearing suited to use under the above-mentioned conditions.
In general, the air dynamic pressure bearing has rigid bearing surfaces on both of a rotary side and a fixed side. However, in the air dynamic pressure bearing of this type, when stability limit is exceeded under a state in which management of radial bearing gaps that are formed between the bearing surfaces on the rotary side and the fixed side is insufficient, self-excited centrifugal whirling of a main shaft, which is called a whirl, is liable to occur. Thus, it is important to manage the gaps in accordance with operating rotation speeds. In particular, in environments involving drastic temperature changes as in the case of the gas turbine and the turbocharger, widths of the radial bearing gaps fluctuate due to thermal expansion, and hence the gaps are significantly difficult to manage with high accuracy.
There has been known a foil bearing as a bearing that is less liable to cause the whirl and allows the gaps to be easily managed even in the environments involving drastic temperature changes. The foil bearing has bearing surfaces formed of flexible thin films (foils) having low flexural rigidity and supports a load by allowing the bearing surfaces to be deflected. Normally, an inner circumferential surface of the bearing is formed of a thin plate called a top foil, and a spring-like member called a back foil is arranged on a radially outer side thereof. With this, a load on the top foil is elastically supported by the back foil. In this case, during rotation of the shaft, an air film is formed between an outer circumferential surface of the shaft and an inner circumferential surface of the top foil. With this, the shaft is supported in a non-contact manner.
The foils of the foil bearing are flexible, and hence appropriate radial bearing gaps are formed in accordance with operating conditions such as a rotation speed of a shaft, a load on the shaft, and an ambient temperature. Therefore, the foil bearing has a feature of excellent stability, and hence can be used at higher speed in comparison with general air dynamic pressure bearings. Further, radial bearing gaps in the general dynamic pressure bearings need to be managed on an order of one thousandth of the diameter of the shaft. For example, in a shaft having a diameter of approximately several millimeters, the radial bearing gaps of approximately several micrometers need to be constantly secured. Thus, in consideration of not only a manufacturing tolerance but also the thermal expansion in the drastic temperature changes, the gaps are difficult to strictly manage. Meanwhile, the foil bearing is advantageous in that radial bearing gaps only need to be managed to have a size of approximately several tens of micrometers, and hence the foil bearing can be easily manufactured and the bearing gaps can be easily managed.
As examples of such foil bearings, there have been publicly known a foil bearing in which the back foil includes cut-and-raised parts so as to elastically support the top foil (Patent Literature 1), a foil bearing in which a bearing foil is elastically supported by an elastic body formed of wires that are woven into a mesh form (Patent Literature 2), a foil bearing in which the back foil includes support portions that are held in contact with an inner surface of an outer race and are immovable in a circumferential direction, and elastic portions that are elastically deflected by contact pressure from the top foil (Patent Literature 3), and the like. Further, in each of Patent Literatures 4 and 5, there is disclosed what is called a multi-arc foil bearing in which a plurality of foils are arrayed in the circumferential direction, and both circumferential ends of each of the foils are mounted to the outer member.